"OpenBSD is developed by volunteers. The project funds development and releases by selling CDs and T-shirts, as well as donations from organizations and individuals. These finances ensure that OpenBSD will continue to exist, and will remain free for everyone to use and reuse as they see fit."

-- www.openbsd.org

Once installed, you don't need to configure it to make it secure. It's secure by default and just works out-of-the-box. It's free. It means that you don't have to pay top-dollars to use it. Moreover you are able to support development by donating money, hardware and sending patches.